The present invention relates generally to electron guns such as used in cathode ray tubes and is particularly directed to a cathode having improved emission characteristics for use in an electron gun.
It is well known that the cathode plays an important role in the emission of electrons in an electron gun used in a cathode ray tube. The structure of a conventional cathode is shown in FIG. 1. A base metal 1 containing Ni as a principal component and a small amount of a reducing agent such as magnesium and silicon is connected to a cathode sleeve 3. A heating device 2 is used for heating an electron emissive layer 4 disposed on base metal 1.
A conventional oxide cathode is made by spraying a suspension on the base metal 1. The suspension is prepared by mixing alkaline earth metal carbonate by ball milling for 24 hours to get the desired particle size and viscosity. After the process of degassing, the coated suspension decomposes to an alkaline earth metal oxide. The reaction can be expressed as follow:
(Ba,Sr,Ca)CO3 xe2x86x92(Ba,Sr,Ca)O+CO2 xe2x80x83xe2x80x83(1)
The alkaline earth metal oxide will react with a reducing agent upon activation.
The reaction can be expressed as follows:
2BaO+Sixe2x86x922Ba+SiO2 xe2x80x83xe2x80x83(2)
xe2x80x83BaO+Mgxe2x86x92Ba+MgOxe2x80x83xe2x80x83(3)
The reducing agent in the base metal 1 diffuses outwardly at a much higher rate to react with the alkaline earth metal oxide to form an electron donor such as free barium. The free barium serves as a source of the electron beam at or in the vicinity of the boundary between the base metal 1 and the electron emissive layer 4.
As reactions (2) and (3) are carried out, another kind of product such as SiO2 will react with the alkaline earth metal oxide continuously to produce an intermediate layer such as of Ba2SiO4. This intermediate layer decreases the outward diffusion rate of the reducing agent in the base metal 1. The rate of producing free barium is decreased and that results in not having enough free barium to produce saturated emission. Because of the limited amount of free barium in a conventional cathode, a conventional cathode can only be used at low current densities, e.g., 0.5-0.8 A/cm2, and at low temperature, e.g., 700xc2x0 C.-800xc2x0 C.
U.S. Pat. No. 4,924,137 discloses an oxide-coated cathode for an electron gun comprising a base metal containing Ni as a major element; a reducing agent disposed in the base; a first electron-emissive layer containing (a) an alkaline earth metal oxide as a principal component containing at least Ba, and (b) a compound of Sc; a second electron-emissive layer formed on the first electron-emissive layer, containing (c) an alkaline earth metal oxide as a principal component also including at least Ba, and (d) at least one heat-resistive oxide selected from the group consisting of oxides of Al, Si, Ti, V, Cr, Fe, Zr, Nb, Hf, Ta, Mo, and W; and a heater for heating the electron-emissive layer. It is further taught that a small amount of metal powder may be added to the electron-emissive layer for improving the conductivity of this layer.
Mitsubishi Co. of Japan has developed a Sc2O3-dispersed oxide cathode with an enhanced current density of up to 2.0 A/cm2. U.S. Pat. No. 5,122,707 discloses this approach wherein Sc2O3 reacts with an intermediate layer of Ba2SiO4 as follows:
Sc2O3+10Nixe2x86x922ScNi5+3Oxe2x80x83xe2x80x83(4)
9Ba2SiO4+16ScNi5xe2x86x924Ba3Sc4O9+6Ba+9Si+80Nixe2x80x83xe2x80x83(5)
The decomposition reaction of Ba2SiO4 with ScNi5 reduces the thickness of the intermediate layer so as not to hinder the reducing agent from diffusing outwardly to react as shown in reactions (2) and (3). A cathode produced in accordance with this approach includes sufficient free barium to produce high current densities.
Based on Sc2O3-dispersed technology, Mitsubishi Co. has further developed a W film-coated oxide cathode capable of producing current densities as high 3.6A/cm2. U.S. Pat. No. 5,118,984 discloses the manufacturing process for this type of cathode. A base metal having a principal component of Ni and a small amount of a reducing agent such as magnesium and silicon is welded to the cathode sleeve. A metal film such as of W or Mo having a thickness less than 2 xcexcm is applied to the base metal in 10xe2x88x925-10xe2x88x928 torr by means of vacuum evaporation or sputtering. The cathode is then heated at 800xc2x0 C.-1100xc2x0 C. in an atmosphere of, for example, hydrogen to remove impurities such as oxygen remaining in the interior or on the surface of the metal layer, and to sinter or recrystallize the metal layer and diffuse the metal layer in the base metal. Finally, a layer of alkaline earth metal oxide comprising 0.01-9 wt. % Sc2O3 is sprayed on the cathode as the electron emissive material. The current density increases due to the W film sputtered on the base metal followed by thermal treatment to form a Ni4W fine crystal structure with the base metal during activation. The intermediate layer of Ba2SiO4 appears to be dispersed by the Ni4W fine crystal structure. This increases the diffusion path between the alkaline earth metal oxide and the reducing agent. The reactions (2) and (3) occur continuously to produce free barium to provide a high current density capability for the cathode.
An indirectly heated cathode according to an embodiment of this invention comprises an electron emissive substance with a two-layer structure. As in a conventional oxide cathode, a base metal has Ni as a principal component with a small amount of a reducing agent such as magnesium and silicon. A two-layer porous electron emissive substance is applied to the surface of the base metal. The outer electron emissive layer is comprised of an alkaline earth metal oxide which may include 0.1-5 wt. % of a rare earth metal oxide. The inner electron emissive layer is comprised of an alkaline earth metal oxide and 1-30 wt. % W and may also further include 0.1-5 wt. % of a rate earth metal oxide. The alkaline earth metal oxide contains barium oxide. The porous electron emissive substance is sprayed on a preheated metal base at the appropriate pressure. The emission current of the inventive cathode is not reduced after life testing of 3000 hours at current densities on the order of 2.0 A/cm2, while the normalized emission current of a conventional oxide cathode typically decreases by approximately 30%, and the normalized emission current of a Sc2O3-dispersed type cathode typically decreases by approximately 14%.